1. Technical Field
This invention relates generally to an aftertreatment system for reducing the amount of NO.sub.x and particulate matter in engine exhaust gases, and more particularly to such a system that oxidizes NO to NO.sub.2 in a first stage, reduces a portion of the NO.sub.2 to N.sub.2 in a second stage, and further reduces a remaining portion of the NO.sub.2 to N.sub.2 and oxidizes carbon to CO.sub.2 in a third stage.
2. History of Related Art
Heretofore, the diesel engine industry has been successful in meeting exceedingly tight emission limits without heavy reliance on expensive exhaust aftertreatment technology. Recently, oxidation catalysts have been used in medium-heavy-duty diesel engine applications to oxidize the soluble organic fraction (SOF) in particulate. Also, even though great strides have been made in controlling the soot emitted from the diesel combustion process, carbonaceous matter in the exhaust stream, generally referred to as particulate matter (PM), continues to make it difficult to meet increasingly stringent proposed particulate limits. Control of soot emissions has been compounded by employing exhaust gas recirculation (EGR) to achieve proposed emission standards.
Using significant EGR rates to achieve very low NO.sub.x emission is forcing some manufacturers to reconsider the use of particulate traps or filters, for their potential in reducing insoluble particulate matter. Oxidation catalysts and traps, with certain limitations on fuel sulfur content, have been combined in a two-stage exhaust aftertreatment system. For example, U.S. Pat. No. 4,902,487 issued Feb. 20, 1990 to Barry J. Cooper, et al. for Treatment of Diesel Exhaust Gases describes an aftertreatment process in which carbonaceous matter is trapped in a filter and oxidized to carbon dioxide. The first stage of the treatment system oxidizes nitrogen oxide (NO) to NO.sub.2 by means of a platinum catalyst. Nitrogen dioxide formed in this stage is then used to oxidize carbon that is trapped in the second stage filter. In this two-stage process, the oxidation of carbon is dependent upon the amount of nitrogen dioxide present in the exhaust stream. If an insufficient amount of NO.sub.2 is present, complete carbon oxidation does not occur.
U.S. Pat. No. 5,746,989 issued May 5, 1998 to Mikio Murachi, et al. for a Method for Purifying Exhaust Gas of a Diesel Engine, describes a method by which NO (nitrogen oxide) in the exhaust gas stream of a diesel engine is oxidized to NO.sub.2 (nitrogen dioxide) by an oxidizing catalyst. Carbon particles in the exhaust gas are trapped by a particulate filter. The exhaust gas containing NO.sub.2 is fed to the particulate filter whereat the NO.sub.2 reacts with carbon particles trapped in the particulate filter and the carbon particles are oxidized. NO.sub.2 is reduced to nitrogen (N.sub.2), H.sub.2 O (water in a gaseous state) and carbon dioxide (CO.sub.2) by enriched fuel mixture combustion in the engine. The oxidizing catalyst uses an electric heater to raise the temperature of the exhaust gas stream to ensure a chemical reaction between the NO.sub.2 with the carbon particles in the particulate filter. Also in the Murachi et al. method, it is necessary to periodically purge the NO.sub.x absorbent. This is accomplished by changing the air-fuel ratio of the engine by injecting fuel into each cylinder of the engine twice during each complete cycle, thus creating a relatively low temperature exhaust gas having a rich air-fuel mixture. This practice generally disadvantageously affects the operating characteristics of the engine.
The present invention is directed to overcoming the problems set forth above. It is desirable to have an aftertreatment system for reducing the amount of nitrogen oxide and particulate matter in engine exhaust gases without the need for electric heating elements to increase the temperature of the exhaust gas to periodically regenerate the particulate filter. It is also desirable to have such an aftertreatment system that does not require the injection of additional fuel in each cylinder of the engine to provide additional necessary hydrocarbon to reduce the NO.sub.x to N.sub.2 (nitrogen). It is desirable to also have an internal bypass for the NO.sub.2 trap to control the rate of NO.sub.2 directed to the carbon trap and, accordingly, the amount of NO.sub.2 bypassed to the carbon particulate filter.